<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div id="app"> {{a}}</div>
    <script src="../dist/vue.js"></script>
    <script>
        // Vue.util.defineReactive
        const vm = new Vue({
            el:'#app',
            data(){ 
                return {a:1} 
            }
        });
    </script>
</body>
</html>